Method of etching copper on cards

ABSTRACT

Etching of copper on a card is achieved by applying an electrical voltage between a cathode ( 102 ) and the card ( 42 ), the card ( 42 ) thereby forming an anode. The cathode ( 102 ) and the card ( 42 ) are immersed in an electrolyte comprising a first component, which may be reduced from a first state in the form of an ion having a metal atom with a first positive oxidation number to a second state in the form of an ion having said metal atom with a second positive oxidation number, which is less than said first positive oxidation number. A first redox potential in the electrolyte for reduction from the first to the second state is larger than a second redox potential in the electrolyte for reduction of divalent copper ions to metallic copper. During the etching metallic copper on the card is oxidized and transferred into positively charged copper ions while the first component is reduced from its first state to its second state. The quality of the etched structures on the card is improved since no metallic copper is precipitated on the cathode.

FIELD OF THE INVENTION

The present invention relates to a method of etching copper on cards orboards, in which method an electric voltage is applied between a cathodeand a board, which constitutes an anode, the cathode and the board beingimmersed in an electrolyte.

The present invention also relates to a device for etching copper onboards, which device comprises a voltage generator, a cathode, which isarranged to be connected to the negative pole of the voltage generator,and a container, which is arranged to contain an electrolyte and,immersed in the electrolyte, the cathode and a board, which is arrangedto be connected to the positive pole of the voltage generator toconstitute an anode.

The invention also relates to an electrolyte for use in electrochemicaletching of copper on boards, in which etching an electric voltage isapplied between a cathode and a board, which constitutes an anode, thecathode and the board being immersed in the electrolyte.

BACKGROUND ART

In the manufacture of patterns, such as electric conductors, on boards,for example copper laminates intended for the manufacture of circuitboards, use is often made of etching to remove portions of anelectrically conductive layer.

The board usually has a sheet of a nonconductive material, which sheetis provided with-a copper layer. on one or on both flat sides. Usually aphotosensitive coating is applied on the copper layer. A photographicprocess is used to transfer a desired pattern from a master or atemplate to the coating, after which the portions of the coatingsurrounding the pattern are removed. In the etching operation, thecopper layer is etched away where the coating has been removed, whereasthe copper layer remains in the portions of the copper layer which arestill covered by the coating. Etching is often carried out aselectrochemical etching, in which a voltage is applied between a cathodeand the board, which constitutes an anode, both the anode and thecathode being immersed in an electrically conductive electrolyte. Thismethod of etching is disclosed, for example, in EP 0 889 680 and WO98/10121. One problem of electrochemical etching is that it is necessaryfor the portions of the copper layer which are to be etched away to bein continuous electrical contact with the voltage source. If, duringetching, a portion of the copper layer loses this electrical contact,this portion will form an “islet” which cannot be etched away. Anotherproblem is that the copper in the copper layer which is oxidised andconverted into copper ions will be reduced on the cathode and formmetallic copper anew. Since this precipitation is not uniformlydistributed over the surface of the cathode, the distance between thecathode and the board will vary over the flat side of the board. Owingto this, also the etching effect will vary over the flat side of theboard, implying that some portions of the board are excessively etchedand some portions are not sufficiently. etched. To obviate theseproblems, the distance between the board and the cathode has to be solong that a change of this distance, due to precipitation of copper,will be insignificant in relation to its magnitude. If a long distanceis provided between the cathode and the board, it is necessary to applya high voltage between the board and the cathode to obtain the desiredcurrent density and thus a sufficient etching rate. However, if a highvoltage is applied, the electric conductors do not obtain the desireddimensions. This in turn prevents the electric conductors on the boardfrom obtaining the intended conductive and resistive properties, whichmakes it necessary to reject a great deal of the etched boards.

SUMMARY OF THE INVENTION

The object of the present invention is to obviate or reduce theabove-mentioned disadvantages and provide a method of etching boards,said method being efficient and resulting in a low defect rate for theetched boards.

More specifically, the invention provides a method of etching copper onboards, in which method an electric voltage is applied between a cathodeand a board, which constitutes an anode, the cathode and the board beingimmersed in an electrolyte, the method being characterised in that theelectrolyte contains at least a first component, which can be reducedfrom a first state in the form of an ion having a metal atom with afirst positive oxidation number, to a second state in the form of an ioncomprising said metal atom with a second positive oxidation number,which is smaller than said first positive oxidation number, a firstredox potential in the electrolyte for reduction from the first to thesecond state being greater than a second redox potential in theelectrolyte for reduction of divalent copper ions to metallic copper,metallic copper on the board being oxidised and converted intopositively charged copper ions while the first component is reduced fromits first to its second state.

Another object of the present invention is to obviate or reduce theabove-mentioned disadvantages and provide a device for etching boards,which device allows efficient etching of boards with a low defect ratefor the etched boards.

More specifically, the invention provides a device for etching copper onboards, which device comprises a voltage generator, a cathode, which isarranged to be connected to the negative pole of the voltage generator,and a container, which is arranged to contain an electrolyte and,immersed in the electrolyte, the cathode and a board, which is arrangedto be connected to the positive pole of the voltage generator toconstitute an anode, said device being characterised in that thecontainer is arranged to receive an electrolyte containing at least afirst component, which can be reduced from a first state in the form ofan ion having a metal atom with a first positive oxidation number, to asecond state in the form of an ion comprising said metal atom with asecond positive oxidation number, which is smaller than said firstpositive oxidation number, a first redox potential in the electrolytefor reduction from the first to the second state being greater than asecond redox potential in the electrolyte for reduction of divalentcopper ions to metallic copper.

It is also an object of the present invention to obviate or reduce theabove-mentioned disadvantages and provide an electrolyte for etchingboards, which electrolyte allows efficient etching of boards with a lowdefect rate for the etched boards.

More specifically, the invention provides an electrolyte for use inelectrochemical etching of copper on boards, in which etching anelectric voltage is applied between a cathode and a board, whichconstitutes an anode, the cathode and the board being immersed in theelectrolyte, the electrolyte being characterised in that it contains atleast a first component, which can be reduced from a first state in theform of an ion having a metal atom with a first positive oxidationnumber, to a second state in the form of an ion comprising said metalatom with a second positive oxidation number, which is smaller than saidfirst positive oxidation number, a first redox potential in theelectrolyte for reduction from the first to the second state beinggreater than a second redox potential in the electrolyte for reductionof divalent copper ions to metallic copper.

Further advantages and characteristics of the invention will appear fromthe description below and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way ofnon-limiting embodiments and with reference to the accompanyingdrawings.

FIG. 1 a is a side view showing a cassette intended for etching a flatside of a copper laminate.

FIG. 1 b shows the cassette in FIG. 1 a along the line Ib-Ib.

FIG. 2 a is a side view showing a cassette intended for etching bothflat sides of a copper laminate.

FIG. 2 b shows the cassette in FIG. 2 a along the line IIb-IIb.

FIG. 3 a is a side view showing another embodiment of a cassette foretching both flat sides of a copper laminate.

FIG. 3 b shows the cassette in FIG. 3 a along the line IIIb-IIIb.

FIG. 3 c shows the area IIIc in FIG. 3 b on a larger scale.

FIG. 3 d shows the cassette in FIG. 3 a along the line IIId-IIId and inan open position.

FIG. 3 e shows an anode frame in the cassette shown in FIG. 3 a.

FIG. 4 a shows a flow-generating pipe, which has a nozzle and which ismoved over a flat side of a cathode.

FIG. 4 b shows the pipe in FIG. 4 a along the line IVb-IVb.

FIG. 4 c shows the pipe in FIG. 4 a along the line IVc-IVc.

FIG. 4 d shows the area IVd in FIG. 4 b on a larger scale.

FIG. 4 e is a cross-sectional view showing an alternative embodiment ofa pipe, which is provided, in its side facing the cathode, with aplurality of nozzles in the form of circular holes.

FIG. 4 f shows the pipe illustrated in FIG. 4 e, seen in the directionof the arrow IVf.

FIG. 5 a shows a flow-generating means in the form of a bar which ismoved over a flat side of cathode.

FIG. 5 b shows the bar in FIG. 5 a along the line Vb-Vb.

FIG. 6 shows an alternative embodiment of a bar.

FIG. 7 shows another alternative embodiment of a bar.

FIG. 8 schematically shows a device for etching copper laminate.

DETAILED DESCRIPTION OF THE INVENTION

In the present description and the appended claims, the term “board”refers to a sheet which has a copper layer. The board can, for example,have a substrate, such as a sheet made of polyester, epoxy plasticreinforced with fiberglass, ceramics or glass, which on one or moresurfaces is provided with a copper layer. One example thereof is copperlaminates, for instance reinforced sheets of epoxy plastic provided withcopper layers and intended for the manufacture of circuit boards for theelectronic industry. “Boards” also refers to sheets made of solidcopper.

In the invention, a board provided with a copper layer is etchedelectrochemically. In this process, a cathode is connected to thenegative pole of a voltage generator, and the copper layer of the boardis connected to the positive pole of the voltage generator and thusbecomes an anode. Both the cathode and the board are immersed in anelectrolyte.

The electrolyte according to the invention has a certain redoxpotential. The redox potential is a measure of the oxidising capacity ofthe electrolyte and depends on the components included, theconcentration of the components and the temperature of the electrolyte.If the redox potential of a certain reaction is lower than the redoxpotential of the electrolyte, the reaction will proceed to the left, asgenerally written and as used in this description. The forms of acomponent included in a reaction of this type are called a redox pair.When several redox pairs are present in the electrolyte, the redox pairwith the highest redox potential will be reduced, i.e. the reaction willproceed to the right, whereas the redox pair with the lowest redoxpotential will be oxidised, i.e. the reaction will proceed to the left.Thus, the redox reaction of copper can be written as follows:Cu²⁺+2e ⁻

Cu E°=0.34 V

In a concentration-dependent activity of 1 for all the componentsincluded and at a temperature of 298 ° K, the redox potential of a redoxpair equals the standard electrode potential, E°, of the redox pair.Table values for standard electrode potentials are available for a greatnumber of redox pairs. Standard electrode potentials are thus quiteuseful when estimating in what direction redox reactions will proceed.The standard electrode potential, E°, of the above reduction of copperis about 0.34 V. In the description below, use is, for practicalreasons, made of standard electrode potentials when comparing the redoxpotentials of different redox pairs. It is important, however, toremember that in a real electrolyte it is the redox potential at thecurrent temperature and in the current activities, and not the standardelectrode potential, that determines the direction and the velocity ofthe reaction.

Besides water, the electrolyte also contains a first. component. Thefirst component has a first state in the form of an ion having a metalatom with a first positive oxidation number, and a second state in theform of an ion comprising said metal atom with a second positiveoxidation number, which is smaller than said first positive oxidationnumber. Said metal atom is preferably Fe, Ce or Mn, and most preferablyFe. Said metal atom can be included in a compound ion or be an ion ofsaid metal atom as such.

When a voltage is applied between the cathode and the anode in the formof the copper layer of the board, electrons will be removed from theanode and brought to the cathode. At the anode, an oxidation of coppertakes place, in which the above-described redox reaction of copperproceeds to the left and copper ions are generated. According tothe-description below, there is no precipitation of copper at thecathode, owing to the nature of the electrolyte.

A first redox potential in the electrolyte for reduction of the firstcomponent from its first state to its second state is higher than asecond redox potential in the electrolyte for reduction of copperaccording to the reaction formula above. Thus, in an electrolytecontaining both copper ions and the first component, the first componentwill be reduced but not the copper ions. As the first component isreduced from a first state in the form of an ion having a metal atomwith a first positive oxidation number, to a second state in the form ofan ion comprising said metal atom with a second positive oxidationnumber, which is smaller than said first positive oxidation number, thereduction of the first component will not result in any precipitation onthe cathode since the second state of the first component is an ion.Both the first and the second state of the first component should beions which are relatively easy to dissolve in the electrolyte. If theseions are easy to dissolve, there is less risk of precipitation of salton the cathode or the anode, even in case of relatively high localconcentrations of ions, which may arise in the vicinity of the anode andthe cathode. Suitably, the solubility of-the first and the second stateof the first component is at least about 0.05 mol/l in the electrolyte.

A first component which can be used in the method according to theinvention is iron, Fe. Fe has a first state in the form of Fe³⁺ and asecond state in the form of Fe²⁺. The redox reaction of this redox paircan be written as follows:Fe³⁺+1e ⁻

Fe²⁺ E°=0.77 V

The standard electrode potential, E°, for the reduction of Fe³⁺ to Fe²⁺is about 0.77 V, i.e. higher than the standard electrode potential forthe reduction of Cu²⁺. In an electrolyte containing both Cu²⁺ and Fe³⁺,Fe³⁺ will be reduced on the cathode to Fe²⁺, whereas Cu²⁺ will remain inion form as long as there is any Fe³⁺ left in the solution. Thus, noprecipitation takes place on the cathode as long as there is any Fe³⁺ inthe electrolyte, which makes it possible to obtain a considerablyshorter and more even distance between the board and the cathode thanhas previously been possible.

In everyday speech, copper is considered as less precious than theso-called red forms which are included in redox pairs having a higherstandard electrode potential than the above-mentioned 0.34 V. When tworedox pairs are mixed in an electrolyte, the direction of the reactionof the redox pair with the higher redox potential will proceed to theright and the redox reaction of the redox pair with the lower redoxpotential will proceed to the left. When metallic copper, Cu, isimmersed in an electrolyte containing the first component in its firststate, the first component will be reduced from its first state to itssecond state while copper is oxidised to copper ions, Cu²⁺, since thefirst redox potential for reduction of the first component from itsfirst state to its second state is greater than the second redoxpotential for reduction of copper ions to metallic copper. Thus, thefirst component will etch copper chemically on the copper layer of theboard. This chemical etching is. independent of the electric voltage. Asa result, the first component helps to etch “islets”, i.e. portions onthe copper layer which are to be etched away but have lost electricalcontact with the positive pole of the voltage generator. This increasesthe amount of high-quality boards and allows etching of more complicatedstructures on the board. Using Fe as the first component, the chemicaletching of copper on the copper layer of the board proceeds according tothe following simplified reaction:Cu+2Fe³⁺

Cu²⁺+2Fe²⁺

According to the invention, use is thus made of simultaneous chemicaland electrochemical etching, in which a first component is used, whichprevents precipitation on the cathode and which can etch copperchemically on the copper layer of the board. Another advantage is thatif any copper, contrary to expectation, should be precipitated on thecathode, the first component will chemically etch also this copper, thuskeeping the cathode clean from precipitation.

Many other compounds can act as a first component. Examples of suchfirst components are manganese, Mn, and cerium, Ce. The redox reactionsfor reduction of these first components from their first states to theirsecond states have the following formulas and standard electrodepotentials, E°:Mn³⁺ +e ⁻

Mn²⁺ E°=about 1.5 VCe⁴⁺ +e ⁻

Ce³⁺ E°=about 1.7 V

Besides Fe, Mn and Ce which are mentioned above, also inorganic andorganic complexes of these substances as well as a plurality of othersubstances are suitable to use as a first component. Furthermore, anumber of different compound ions can be employed. Examples of suchfirst components and their redox reactions are:MnO₄ ⁻+8H⁺+5e ⁻

Mn²⁺+4H₂O E°=1.51 VCr₂O₇ ²⁻+14H⁺+6e ⁻

2Cr³⁺+7H₂O E°=1.33 V

In the above examples, it is thus a metal atom, namely Mn or Cr.included in the ion, that is converted in the reduction from a firstpositive oxidation number to a second positive oxidation number, whichis smaller than said first. positive oxidation number.

By studying tables of standard electrode potentials of redox pairs, itis possible to find +more compounds which can be used as a firstcomponent. Examples of organic complexes are ferric bipyridines, [Fe(bipy)₂]³⁺ and [Fe (bipy)₃]³⁺:[Fe(bipy)₂]³⁺ +e ⁻

[Fe(bipy)₂]²⁺ E°=0.78 V[Fe(bipy)₃]³⁺ +e ⁻

[Fe(bipy)₃]²⁺ E°=1.03 V

Preferably, the first component is not injurious, or is only moderatelyinjurious, to the health and/or the environment in its first and itssecond state. The first component is also preferably commerciallyavailable at a low cost.

The etching according to the invention is both electrochemical andchemical. Preferably, most of the etching is electrochemical since thisreduces the under etching. Approximately about 70-90%, preferably about80%, of the copper that is removed from the copper layer of the boardshould be removed by electrochemical etching, the remaining about30-10%, preferably about 20%, of the copper being removed by chemicaletching. Therefore, the chemically etching effect of the electrolyteshould not be unnecessarily powerful. The chemical etching rate isdefined as the thickness reduction per time unit of a copper layer whichis etched chemically. It has been found that the chemical etching rate,i.e. the capacity of the electrolyte to etch the copper layer in theabsence of an applied voltage, should be about 6-100 nm/s, preferablyabout 10-70 nm/s. The desired chemical etching rate depends on the sizeof the electric conductors which are to be made on the board. Whenetching boards with electric , conductors having a width of about 50 μm,the thickness of the copper layer usually being about 18-35 μm or less,a lower chemical etching rate is usually preferred, such as 10-50 nm/s,more preferably about 30 nm/s. In electric conductors having a width ofabout 20 μm, an electrolyte with an even lower chemical etching rate,such as 10-20 nm/s should be used.

The concentration of the first component is suitably about 0.02-0.7mol/l, preferably about 0.05-0.5 mol/l. The concentration must beselected in consideration of the redox potential for reduction of thefirst component from its first to its second state and the desiredchemical etching effect. It has thus been found that, when the firstcomponent is in the form of Fe, it is convenient to use a concentrationof about 0.1-0.5 mol/l Fe³⁺. When etching small structures, such as theabove-mentioned electric conductors with a width of 50 μm and eventhinner conductors, the concentration of Fe can suitably be about0.05-0.3 mol/l Fe³⁺.

The greater the difference between the first redox potential in theelectrolyte for reduction of the first component from its first to itssecond state and the second redox potential in the electrolyte forreduction of divalent copper ions to metallic copper, the lower theconcentration of the first component should be for the chemical etchingrate not to be excessively high. Thus, when using Ce, the concentrationof Ce⁴⁺ should be much lower than the concentration of Fe³⁺ when usingFe. In case of a lower concentration of the first component, the numberof ions moving in the electrolyte will decrease. This results inpolarisation, which means that there is a shortage of positively chargedions at the cathode and a shortage of negatively charged ions at theanode. A high degree of polarisation necessitates an increase in theapplied voltage to provide sufficient current density and thus asufficient etching rate. However, when increasing the voltage, theabove-mentioned problems concerning the dimensions of the conductorswill occur.

Preferably, the first redox potential should be less than about 1.0 V,but more than about 0.4 V, to provide a suitable-chemical etching rate.In this context, a particularly preferred component is Fe. The redoxpotential for the reduction of Fe³⁺ to Fe²⁺ at the current temperaturesand concentrations is relatively low. Consequently, it is possible tomaintain a relatively high concentration of Fe³⁺, which reduces thepolarisation, without an excessive increase in the etching effect. Fe³⁺is a small ion which is mobile and takes up electrons efficiently fromthe cathode, thus reducing the polarisation.

In some cases, it may be interesting to use two different firstcomponents. When using a first component, such as Ce, which is highlyoxidising in its first state, for example Ce⁴⁺ in the case of Ce, it maysometimes be convenient to add one more first component, such as Fe. Fecan be added in the form of Fe³⁺ or Fe²⁺. The chemically etching effectof Fe³⁺ in the presence of Ce⁴⁺ is very limited. However, because of itscharge and the fact that it can be added in higher concentrations thanCe⁴⁺ without causing an excessively high redox potential in theelectrolyte, Fe³⁺ will reduce the polarisation in the electrolyte. As aresult, the voltage can be reduced at a given desirable current densityin the electrolyte, which reduces the underetching.

It is often suitable to add a second component to the electrolytetogether with the first component. The second component can be reducedfrom a first state in the form of an ion having an atom with a firstpositive oxidation number, to a second state in the form of an ioncomprising said atom with a second positive oxidation number, which islower than said first positive oxidation number, a third redox potentialin the electrolyte for reduction of the second component from its firststate to its second state being lower than the second redox potential inthe electrolyte for reduction of divalent copper ions to metalliccopper. Thus, the second component cannot etch copper. On the otherhand, the second component has a positive effect on the migration ofions in the electrolyte. As a result, the second component reduces thepolarisation in the electrolyte, which makes it possible to provide thedesired-current density at a lower voltage, thus reducing theunderetching. Suitable second components are those that are mobile inthe electrolyte and efficiently reduce the polarisation. The secondcomponent should thus have a high electric charge, a small size andshould not associate with water or precipitate. Preferably, the secondcomponent is Sn. Sn can be present as Sn⁴⁺, which can be reduced to Sn²⁺according to the following redox reaction:Sn⁴⁺+2e ⁻

Sn²⁺ E°=0.15 V

The second component can thus take up electrons on the cathode and bereduced. However, these electrons will be given, in the bulk ofelectrolyte, to the first component, which is then reduced from itsfirst to its second state, while the second component is oxidised to itsfirst state. Consequently, the second component seems to act as anelectron carrier. Since the third redox potential for converting thesecond component from its first to its second state is lower than thesecond redox potential for reduction of copper ions to metallic copper,there is no risk of precipitation of the second component since bothcopper ions and the first state of the first component will oxidise thesecond state of the second component, which then converts to the firststate. Naturally, a third state of the second component, which state canbe uncharged or constitute a positively charged ion having a lowercharge than the second state of the second component, will also beoxidised by both copper ions and the first state of the first component.

The concentration of the second component is suitably about 0.005-0.4mol/l, preferably about 0.05-0.2 mol/l, and more preferably about 0.1mol/l.

The solubility of Sn²⁺ is limited in the electrolyte, which means thatthe concentration of Sn²⁺ has to be kept very low. It has, however, beenfound that an addition of hydrogen fluoride, HF, to the electrolyteincreases the solubility of Sn in such a manner that an amount of Snsuitable for reducing the polarisation can be present in the electrolytewithout any precipitation of Sn, for example in the form of SnO₂. Thereason for this is that SnF₄ is very easily dissolvable. It has beenfound that the electrolyte suit ably has a concentration of about0.01-0.5 mol/l F⁻. It has been found that in an electrolyte containingHF, according to that described above, it is suitable to use aconcentration of Sn of about 0.005-0.4 mol/l. An addition of HF reducesthe pH, which increases the solubility of many ions, Fe³⁺ among others.Another advantage of HF is that small and mobile ions in the form of H⁺and F⁻ are added to the electrolyte, which reduces the polarisation.

It has also been found that an addition of HF has apolarisation-reducing effect, even when no Sn is present in theelectrolyte. This means that fluoride ions, F⁻, can be added to theelectrolyte, for example in the form of HF or NaF, to reduce thepolarisation, irrespective of whether Sn is present in the electrolyteor not.

The electrolyte has to contain negatively charged ions, which arecounter ions of the positively charged ions in the electrolyte. Thesenegatively charged ions should have good solubility for the copper ionspresent in the electrolyte as well as for the positively charged ions ofthe first component and, if present, the second component. To avoidprecipitation, solubility limits for salts, etc, of ions contained inthe electrolyte should not be exceeded.

Furthermore, the negatively charged ions should not cause any generationof gas during the electrochemical etching, nor should they etch thecopper. Examples of suitable counter ions are sulphate ions, SO₄ ²⁻, andfluoride ions, F⁻, which both have good solubility for copper ions andiron ions. In particular F⁻ is a very small and mobile ion, whichthereby helps reducing the polarisation in the electrolyte duringetching. In some cases, the chloride ion, Cl⁻, is a less suitablecounterion, as it tends to cause uneven etching and pitting, i.e.undesirable pit erosion in metal objects, for example electricconductors, wires and the like, immersed in the electrolyte.

It has, however, surprisingly been found that if fluoride ions, F⁻, arepresent in the electrolyte, the quality of the etched structures, forexample the electric conductors, can be improved by adding also chlorideions, Cl⁻, to the electrolyte. It is particularly preferred for theelectrolyte to contain both F⁻, Sn⁴⁺ and Cl⁻. A suitable concentrationof chloride ions, Cl⁻, in an electrolyte, to which also HF has beenadded, is 0.03-1.5 mol/l, preferably 0.1-0.5 mol/l. The chloride ions,Cl⁻, can, for example, be added to the electrolyte in the form of NaClor hydrochloric acid, HCl.

In some cases, it may be suitable to add more compounds to furtherreduce the polarisation without etching the copper layer. Examples ofsuch suitable compounds are Cu(BF₄)₂, Sn(BF₄)₂ and HBF₄. These compoundscan suitably be added to the electrolyte in a concentration of about0.05-0.4 mol/l.

A fourth redox potential in the electrolyte for reduction of the firstcomponent from said second state to a third state is lower than saidsecond redox potential. As a result, the second state of the firstcomponent cannot etch copper chemically by reduction to the third stateof the first component, which is advantageous since the chemicallyetching effect should not, as already mentioned, be excessively high. Ifthe third state of the first component is an uncharged form, it is anadvantage, should any precipitation occur on the cathode, if it is aprecipitation of copper and not of the first component, which is kept inthe solution. It is also preferable for said fourth redox potential tobe lower than the redox potential for the generation of hydrogen gas. Ifthe electrolyte has been unintentionally depleted of the first componentin its first state and of copper ions, hydrogen gas generation willprevent precipitation of the first component on the cathode. Redoxpotentials for reduction of the first components mentioned above fromtheir second to their third state as well as the redox potential forhydrogen gas generation can thus be written as follows:Fe²⁺+2e ⁻

Fe E°=−0.44 VCe³⁺+3e ⁻

Ce E°=−2.3 VMn²⁺+2e ⁻

Mn E°=−1.2 V2H⁺+2e ⁻

H₂ E°=0.0 V

Thus, neither Fe²⁺, Ce³⁺ nor Mn2+ can etch copper on the board, andthere will be no precipitation of Fe, Ce or Mn on the cathode.

The pH of the electrolyte should preferably be about −0.5 to 4, and morepreferably about 0 to 2. A lower pH improves the solubility of thecomponents included and reduces the polarisation owing to the increasedconcentration of the mobile hydrogen ions. The pH can, for example, beadjusted by means of sulphuric acid, H₂SO₄. The temperature. of theelectrolyte is usually set at about 20-50° C. When etching smallstructures on the board, such as electric conductors having a width ofabout 50 μm and less, the temperature should suitably be about 20-35°C., preferably about 30° C. When etching larger structures, thetemperature can suitably be higher, such as 35-50° C., since a highertemperature increases the chemical etching rate.

In a method of etching a board, the board and a cathode are immersed inan electrolyte of the kind described above. The copper layer on a flatside of a board is connected to the positive pole of a voltage generatorso as to constitute an anode and the cathode is connected to thenegative pole of the voltage generator. A voltage of about 0.5-6 V,preferably about 1-3 V, is then applied between the copper layer and thecathode, the board being etched electrolytically as well as chemicallywhile measuring the electric current. When after a certain period oftime the electric current decreases, which indicates that a great dealof the copper layer has been etched away, the voltage generator isturned off. As an alternative, the etching takes place at a constantelectric current while measuring the voltage, the electric current beingturned off as the potential rises. The board is then allowed to remainstill longer in the electrolyte, while final etching in the form of apurely chemical etching takes place.

According to a preferred method, a flow of electrolyte is removed,preferably continuously, from the container in which the board and thecathode are immersed in the electrolyte. The flow which has been removedis conducted to a regeneration tank. In the regeneration tank, aregenerant is added, which can oxidise the first component from itssecond to its first state. The regenerant thus has a fifth redoxpotential for reduction which is greater than the first redox potentialfor reduction of the first component from its first state to its secondstate. One example of a suitable regenerant is sodium persulphate,Na₂S₂O₈. The redox reaction of the persulphate ion in Na₂S₂O₈ is asfollows:S₂O₈ ²⁻+2e ⁻

2SO₄ ²⁻E°=2.01 V

In the contact with a first component, whose first redox potential islower than the above-mentioned fifth redox potential, S₂O₈ ²⁻ will thusbe reduced while the first component is oxidised from its second to itsfirst state. If the first component is Fe, the following reaction willtake place:2Fe²⁺+S₂O₈ ²⁻

2Fe³⁺+2SO₄ ²⁻

When the first component has been converted from its second to its firststate, the regenerated electrolyte is pumped back to the container, inwhich the board and the cathode are immersed in electrolyte. Theregeneration of the electrolyte can also be carried out batchwise,either in a special regeneration tank or in the container intended tocontain the board and the cathode.

As mentioned above, said fifth redox potential for reduction of S₂O₈ ²⁻is so high that S₂O₈ ²⁻ is able to oxidise also Mn²⁺ to Mn³⁺ and Ce³⁺ toCe⁴⁺. It will be understood that, in principle, S₂O₈ ²⁻ can be used as afirst component when etching the copper layer of the board. In mostcases, however, S₂O₈ ²⁻ yields too high a chemical etching effect to beuseful as a first component. Nor is it possible to regenerate S₂O₈ ²⁻ ina simple way.

In the reduction of S₂O₈ ²⁻, two sulphate ions, S₄ ²⁻, are formed. Thisis an advantage since these sulphate ions can act as counter ions to thecopper ions generated in the etching of the copper layer of the board.Copper sulphate has a high solubility and therefore a certain amount ofelectrolyte can be used for a relatively long period of time before theconcentration of copper sulphate gets too high. Some of the electrolyteis suitably continuously drained so as to keep the copper sulphatecontent below the solubility limit, while water and the first and secondcomponents in the electrolyte are added to compensate for the drainedelectrolyte. Since the copper sulphate content is normally higherlocally at the surface of the board, where precipitation of coppersulphate would be injurious, it is often suitable not to allow thecopper ion content in the bulk of electrolyte to exceed about 1 mol/l.

The first and the second component can be added in the form of saltscontaining the first and the second components in their first or secondstate. For example, Fe can be added in the form of Fe³⁺ as Fe₂(SO₄)₃ orin the form of Fe²⁺ as FeSO₄. In the latter case, FeSO₄ is suitablyadded to the regeneration container and treated with a regenerant, suchas Na₂S₂O₈, for converting Fe²⁺ to Fe³⁺, which is subsequently used inetching. Thus, FeSO₄ functions as a precursor to the formation of Fe³⁺.

Samples of the electrolyte are taken and analysed as to the content ofthe first component in its first state. When Fe is used as the firstcomponent, the Fe²⁺ content can be analysed by titration withpermanganate ions. From this analysed Fe²⁺ content and from the known,original Fe³⁺ content, the current amount of Fe³⁺ can be calculated. Inthis way, it is ensured that a sufficient concentration of the firstcomponent in its first state is always available in the container inwhich the board is etched. A sufficient concentration of the firstcomponent in its first state is necessary in order to avoidprecipitation of copper on the cathode and to provide the desiredchemical etching effect. Excess use of regenerant should, however, beavoided, since this can cause a chemical etching effect that isunnecessarily powerful. The addition of regenerant to the regenerationtank is controlled on the basis of the analysis of the content of thefirst component in its first state. It is also possible to introduceautomatic sampling and/or control of the addition of regenerant.

The applied voltage during etching is selected according to thestructure that is to be etched, the nature of the electrolyte and thedistance between the cathode and the board. In small structures, such aselectric conductors with a width of 50 μm and less, use isadvantageously made of a low voltage, such as a voltage of 0.5-3 V,since the effects of under etching are more significant in thinconductors. In addition, the closer the copper layer of the board islocated to the cathode, the lower voltage is needed for a certainetching rate. It is thus an advantage for the distance between thecathode and the board to be kept as short as possible, in particularwhen small structures are being etched. According to the invention,precipitation on the cathode is avoided, and the distance between thecathode and the board can be kept small and even over the flat side ofthe board during the entire etching process. It is, however, alsopossible, when etching very large boards, to increase the distancebetween the cathode and the board. The reduced polarisation provided bythe electrolyte makes it possible to increase the distance between thecathode and the board compared to prior art technique, while using thesame voltage and current density. As a result, the degree of sensitivityto irregularities in the board and the cathode is reduced in such largeboards without an increase in the underetching.

EXAMPLE 1

A plurality of electrolyte mixtures were prepared of water and thecompounds described below. In all cases but one, use was made of 100 gsalt per liter of water. A cathode and an anode were immersed in theelectrolyte at a distance of 1 cm from each other and a voltage of 2.0 Vwas applied between the cathode and the anode. The anode had the form ofa circular surface with a diameter of 2 cm. Table 1 shows the electriccurrent that was measured in the different electrolyte mixtures.

TABLE 1 Currents measured in different electrolyte mixtures. Compoundsadded to water Current (A) FeSO₄ 0.09 Fe₂(SO₄)₃ 0.23 CuSO₄ 0.17 Na₂S₂O₈0.26 CuSO₄ + Fe₂(SO₄)₃ + 0.40 Sn(SO₄)₂ + HF * * This electrolyte mixturecontained 20 g CuSO4, 80 g Fe₂(SO₄)₃, 8 g Sn(SO₄)₂ and 15 g HF per litreof water.

As appears from Table 1, an electrolyte containing Cu²⁺, Fe³⁺, Sn⁴⁺ andHF provides significantly higher current and thus lower polarisationthan an electrolyte containing copper ions only.

EXAMPLE 2

A board in the form of a copper laminate intended for the manufacture ofcircuit boards had a size of 2.5×2.5 cm. The copper laminate had on aflat side a copper layer with a thickness of 18 μm, on which a photosensitive coating was applied. A photographic process was used totransfer a pattern from a master to the coating. The pattern included 50μm lines/gaps, i.e. lines, for example electric conductors, and gapswith a width of about 50 μm. An electrolyte was prepared by dissolvingFe₂(SO₄)₃ and CuSO₄ in water to a concentration of 0.25 mol/l Fe³⁺ and0.05 mol/l Cu²⁺. The pH of the electrolyte was adjusted by means ofsulphuric acid to pH=0.6. The copper laminate and a sheet-like cathodewere then placed in a container holding said electrolyte. A voltage of 2V was applied between the copper laminate and the cathode, and thecurrent intensity was measured to about 2 A. The amount of Fe³⁺ in theelectrolyte corresponded to a great excess in relation to the amount ofcopper that was to be etched, which made regeneration unnecessary duringthe etching process. The distance between the flat side of the copperlaminate and the flat-side of the cathode facing the board was 8 mm. Thetemperature of the electrolyte was 35° C. The board was etched in thismanner for about 100 s. As the current began to drop, the voltage wasturned off, and the copper laminate was washed to remove the electrolyteand the remaining coating.

When examining the flat side of the cathode facing the copper laminate,no precipitation of copper or any other substances was found. Whenexamining the copper layer of the copper laminate under a microscope, itwas found that the sides of the lines which had been etched on thecopper laminate were somewhat frayed. No traces of “islets” of coppercould be found on the copper laminate.

EXAMPLE 3

A copper laminate was etched under the same experimental conditions asin Example 2, except that NaCl was added to the electrolyte in such anamount that the concentration of Cl⁻ was 0.25 mol/l.

No precipitation was observed on the cathode. When examining the etchedcopper laminate under a microscope, it was found that the sides of thelines that had been etched were to a large extent frayed and in someplaces the lines were completely broken. Traces of “islets” of coppercould also be observed on the copper laminate. An addition of chlorideions thus seems to deteriorate the etching result.

EXAMPLE 4

A copper laminate was etched under the same experimental conditions asin Example 2, except that NaCl in such an amount that the concentrationof Cl⁻ was 0.25 mol/l and HF in such an amount that the concentration ofF⁻ was about 0.05 mol/l were also added to the electrolyte.

No precipitation was observed on the cathode. When examining the copperlayer of the copper laminate under a microscope, it was found that thesides of the lines that had been etched on the copper laminate werequite smooth. The lines were not broken and no “islets” of copper couldbe found. The upper sides of the lines, however, showed signs ofunderetching, i.e. the electrolyte had etched copper under the coating.Accordingly, an addition of chloride ions and fluoride ions to theelectrolyte improves the etching result.

EXAMPLE 5

A copper laminate was etched under the same experimental conditions asin Example 4, except that Sn⁴⁺ in a concentration of about 0.07 mol/land (BF₄)⁻ in a concentration of about 0.14 mol/l were also added to theelectrolyte.

No precipitation was observed on the cathode. When examining the copperlayer of the copper laminate under a microscope, it was found that thesides of the lines that had been etched on the copper laminate werequite smooth. The lines were not broken and no “islets” of copper couldbe found. The upper sides of the lines showed limited signs of underetching and had a very high quality. Accordingly, the electrolyte usedin Example 5 yields an excellent etching result.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 a and 1 b show a cassette 1 for a board in the form of a copperlaminate 2 to manufacture a circuit board. The copper laminate 2 has anelectrically non-conductive base 4 made of polyester. On a flat side 6,the copper laminate 2 is provided with a copper layer 8, which has athickness of about 35 μm. The portions of the copper layer 8 which areto constitute electric conductors on the finished circuit board arecoated with a thin coating in the form of a thin resist layer 10, whichprotects these portions of the copper layer 8 during etching. Thecassette 1 has a cathode 12 in the form of electrically conductive metalnetting made of acid-proof steel, such as SS 2343, which cathode 12 iswelded on a cathode frame 14, which is made of a sectional element ofacid-proof steel, in such a manner that the cathode frame 14 has goodelectrical contact with the cathode 12. An insulator 16 in the form of alayer of electrically insulating epoxy adhesive reinforces theattachment of the cathode 12 to the cathode frame 14, protects thenetting, prevents the weld joint from being etched and insulates thecathode 12 from an anode frame 18, which is made of a sectional elementof acid-proof-proof steel. The anode frame 18 is pressed against thecopper layer 8 of the copper laminate 2 so as to have good electricalcontact with the copper layer 8. A support frame 20 is pressed againstthe base 4 of the copper laminate 2. A plurality of clamps 22, which aretightened by means of screws 24 and are made of an electricallyinsulating material, such as PEEK (polyetheretherketone plastic), whichcombines very high hardness with good electrically insulatingproperties, press the support frame 20, and thus the copper laminate 2,against the anode frame 18, the insulator 16, the cathode 12 and thecathode frame 14. Both the insulator 16 and the anode frame 18 have awell-defined thickness, which makes the distance H between the copperlayer 8 of the copper laminate 2 and the cathode 12 constant over theentire surface of the flat side 6 of the copper laminate 2.

A voltage generator 26 supplies a cathode line 28 with negative voltageand an anode line 30 with positive voltage. The cathode line 28 providesthe cathode frame 14, and thus the cathode 12, with negative voltage.The anode line 30 provides the anode frame 18, and thus the copper layer8 of the copper laminate 2, with positive voltage.

FIGS. 2 a and 2 b show a cassette 40 for etching a board in the form ofa copper laminate 42 to manufacture a circuit board The copper laminate42 has an electrically non-conductive base 44 made of polyester. On afirst flat side 46, the copper laminate 42 has a copper layer 48 with athickness of about 35 μm. The portions of the copper layer 48 which areto constitute electric conductors on the finished circuit board arecoated with a thin resist layer 50. On its second flat side 52, thecopper laminate 42 has a copper layer 54 with a thickness of about 35μm. The portions of the copper layer 54 which are to constitute electricconductors on the finished circuit board are coated with a thin resistlayer 56. The cassette 40 has a first cathode 58, which is of the samekind as the cathode 12. The first cathode 58 is welded on a firstcathode frame 60, which is of the kind described above in connectionwith the cathode frame 14. A first insulator 62 in the form of a layerof electrically insulating epoxy adhesive reinforces the attachment ofthe first cathode 58 to the first cathode frame 60, protects the weldjoint and insulates the first cathode 58 from a first anode frame 64,which is made of a sectional element of acid proof steel. The firstanode frame 64 is pressed against the copper layer 48 on the first flatside 46 of the copper laminate 42 so as to have good electrical contactwith the copper layer 48. The cassette 40 also has a second cathode 66,which is of the same kind as the cathode 12. The second cathode 66 iswelded on a second cathode frame 68, which is of the kind describedabove in connection with the cathode frame 14. A second insulator 70 inthe form of a layer of electrically insulating epoxy adhesive reinforcesthe attachment of the second cathode 66 to the second cathode frame 68,protects the weld joint and insulates the second cathode 66 from asecond anode frame 72, which is made of a sectional element of acidproof steel. The second anode frame 72 is pressed against the copperlayer 54 on the second flat side 52 of the copper laminate 42 in such amanner that the anode frame 72 has good electrical contact with thecopper layer 54. A plurality of clamps 22 of the above-described kindpress the cathode frames 60, 68, the cathodes 58, 66, the insulators 62,70 and the anode frames 64, 72 against the copper laminate 42. Theinsulators 62, 70 and the anode frames 64, 72 have a well definedthickness, which makes the distance H1 between the copper layer 48 andthe first cathode 58 and the distance H2 between the copper layer 54 andthe second cathode 66 constant over the entire surface of the flat sides46 and 52, respectively, of the copper laminate 42. However, H1 and H2need not be the same distance but can be adjusted individually dependingon the desired etching of the respective flat sides 46, 52.

A first voltage generator 74 supplies a first cathode line 76 withnegative voltage and a first anode line 78 with positive voltage. Asecond voltage generator 80 supplies a second cathode line 82 withnegative voltage and a second anode line 84 with positive voltage. Thecathode lines 76, 82 supply the respective cathode frames 60, 68, andthus the respective cathodes 58, 66, with negative voltage. The anodelines 78, 84 supply the respective anode frames 64, 72, and thus therespective copper layers 48, 54 of the copper laminate 42, with positivevoltage. The two voltage generators 74, 80 can feed either the samevoltage or a voltage that is individually adjusted to the etching of therespective copper layers 48, 54. Also the time of interrupting theetching can be individually adjusted for the respective voltagegenerators 74, 80. If it is desirable to have the same voltage and timesetting, one voltage generator only can feed negative voltage to boththe cathode lines 76, 82 and positive voltage to both the anode lines78, 84.

FIGS. 3 a-3 e show another embodiment of a cassette 100 for etching theflat sides 46 and 52 of a copper laminate 42 of the type shown in FIG. 2b. The cassette 100 has a first cathode 102 and a second cathode 104 inthe form of metal netting, which have been attached in theabove-described manner to a first and a second cathode frame 106 and108, respectively. The cathodes 102, 104 are fixed in the respectivecathode frames 106, 108, which means that the surface of the cathodes102, 104 is quite smooth and precise distances can be obtained in thecassette 100 between the respective cathodes 102, 104 and the respectiveflat sides 46, 52. A first and a second glue thread 110 and 112,respectively, of electrically non-conductive epoxy adhesive insulate thecathode 102 and 104, respectively, and the cathode frame 106 and 108,respectively, from a first and a second anode frame 114 and 116,respectively. The glue threads 110, 112 also protect the cathodes 102,104 and the weld joints against etching. Each cathode frame 106, 108 hasa groove 118 and 120, respectively, in which a first and a secondinsulator in the form of a first and a second circumferential,electrically insulating strip 122 and 124, respectively, made of PEEKare arranged. The first cathode frame 106 and the first anode frame 114are screwed on the first strip 122 each from one side in such a mannerthat a gap 126 is provided between the cathode frame 106 and the anodeframe 114, and there is no electrical contact between these frames 106,114. A gap 128 is formed in a similar manner between the second cathodeframe 108 and the second anode frame 116. Unlike the embodiments shownin FIGS. 1 a-1 b and 2 a-2 b, the insulating space in the form of thegap 126, 128 between the cathode frame 106, 108 and the anode frame 114,116 is not provided by the glue thread 110, 112, but by the electricallyinsulating strip 122, 124. It is easier to shape the strip 122, 124 thanthe glue thread 110, 112 into close tolerance, which means that thedistance between the cathode 102, 104 and the flat side 46, 52 of thecopper laminate 42 will be precise and equal over the entire surface ofthe copper laminate 42.

The cathode 102, cathode frame 106, the insulator 110, the strip 122 andthe anode frame 114 form a first part 130, whereas the cathode 104, thecathode frame 108, the insulator 112, the strip 124 and the anode frame116 form a second part 132. The parts 130, 132 are joined together inthe lower portion of the cassette 100, as seen in FIG. 3 d, by means ofhinges 134, 136, which are made of an electrically insulating material,such as PEEK. The hinges 134, 136 can also be made of the same material,for example acid proof steel, as the rest of the cathode frames 106,108, and if different voltage is to be applied to the cathodes 102, 104,the hinges are suitably insulated from the cathode frames.

Each strip 122, 124 is provided with a plurality of circular holes 138,140. Neodym magnets 142, 144, which are also called NdFeB, are insertedinto the holes 138, 140. The magnets 142 in the strip 122 attract themagnets 144 in the strip 124 without getting into contact with them, thefirst part 130 being able to be joined with the second part 132 withoutany electrical contact arising between the first and the second part 130and 132, respectively. A handle 146 located in an upper portion of thecassette 100, as seen in FIG. 3 a, is used to separate the first part130 from the second part 132 and to open the cassette 100, when thecopper laminate 42 is to be removed from the cassette 100. FIG. 3 dshows the cassette 100 in an open position. FIGS. 3 b and 3 c show thecassette 100 in a closed position with an inserted copper laminate 42.

A voltage generator (not shown) supplies a first cathode line 148 and asecond cathode line 150 with negative voltage, and a first anode line152 and a second anode line 154 with positive voltage. The cathode lines148, 150 provide the respective cathode frames 106, 108, and thus therespective cathodes 102, 104, with negative voltage. The anode lines152, 154 provide the respective anode frames 114, 116, and thus therespective copper layers 48, 54 of the copper laminate 42, with positivevoltage.

Both the cathode lines 148, 150 and the anode lines 152, 154 are made ofhollow, square sectional elements 151 made of acid proof steel andprovided with an inner core 153 of copper to obtain sufficientconductivity. The square sectional elements 151, which thus protect therespective copper cores 153 from being etched, are welded to the cathodeframes and the anode frames, respectively, to provide good electricalcontact. The second cathode frame 108 is provided with a first and asecond guiding means 156, 158. The guiding means 156, 158 are slidablymounted in vertical slides 160, 162, which allows the cassette to beeasily moved out of and into an electrolyte bath.

The anode frames 114 and 116 have at one side, as shown in FIG. 3 e withreference to the anode frame 114, a grooved portion 164 extending alongthe inner edge of the anode frame. The grooved portion 164, which has awidth BA of about 10 mm, is intended to engage the copper layer 48, 54of the copper laminate 42 in such a manner that the copper laminate 42is securely fastened and that good electrical contact is obtainedbetween the respective anode frames 114, 116 and the respective copperlayers 48, 54 along the entire periphery of the copper laminate 42.Owing to the firm fixture of the copper laminate 42 between the anodeframes 114, 116 along the entire periphery, also large copper laminates42 and copper laminates 42 with low rigidity can be etched at distancesH1, H2 that are very small yet constant over the entire first and secondflat sides 46 and 52, respectively, of the copper laminate 42. As shownin connection with the anode frame 114 in FIG. 3 e, the anode frames 114and 116 also have a plurality of screw holes 166, which are intended forscrewing the anode frame 114 and 116, respectively, to the electricallyinsulating strip 122 and 124, respectively. The anode frames 114 and 116also have a plurality of recesses 168, which enable the magnets 142 and144, respectively, holding the parts 130, 132 together to get so closeto each other that the parts 130 and 132 are kept well-together. Thestrip 122, 124, which is otherwise flat, thus has projections 123 and125, respectively, provided with holes 138 and 140, respectively, inplaces corresponding to the recesses 168, as shown in FIG. 3 c. At oneend 170, each anode frame 114 and 116 has a plurality of connectingflanges 172, to which the anode line 152 and 154, respectively, is to bewelded.

It is often suitable to produce a plurality of anode frames 114, 116,according to FIG. 3 e, of sheet steel with different thickness. As anexample, anode frames 114, 116 with a thickness of 2, 5, 7 and 10 mm canbe made. By means of a set of such anode frames the distances H1, H2, ina given cassette 100, can easily be changed by screwing an anode frame114, 116 of suitable thickness to the respective strips 122, 124. Theopening 165 of the anode frame 114, 116 corresponds to the portion ofthe copper laminate 42 that is to be etched. By manufacturing anodeframes with a smaller opening 165, also copper laminates with a smallerflat side can be etched in the cassette 100.

The cathode 12, 58, 66, 102, 104 is made of electrically conductivemetal netting, which is preferably made of acid-proof steel. The nettingis suitably made of metal threads with a diameter of about 300-1000 μm,which have been woven together into netting, preferably in a squarepattern. If just a few or thin threads are used, the resistance in thenetting increases, which results in generation of heat and reduction ofthe current that can be passed in the netting. If too thick threads areused, the electrolyte flow through the netting decreases. The openings13, 59 in the form of meshes formed between the threads should be about0.3-2 mm in a square. In case of too small openings, the netting risksgetting clogged at the same time as the electrolyte flow through thenetting decreases. In case of too large openings, the number of threadsis reduced, which increases the resistance. This also makes the etchingof the copper layer 8, 48, 54 more uneven, the etching usually beingmore powerful under the actual threads.

FIGS. 4 a-4 d show a first embodiment of a flow-generating means in theform of a pipe 200. As seen in FIG. 4 b, the cathode 12 which. is formedas netting has a first flat side 202 facing the copper laminate 2 and asecond flat side 204 facing away from the copper laminate 2. The pipe200, which is a hollow, square sectional element, is attached to adisplacing means in the form of a rail 206, which is arranged to movethe pipe 200 along the second flat side 204 of the cathode 12 in thedirection of the arrow P. When the pipe reaches a first end position208, the rail 206 changes the direction of movement and moves the pipe200 in the opposite direction towards a second end position 210. A pump212 is arranged to supply the pipe 200 with electrolyte by means of aflexible hose 213 during the entire movement. It is important for themovement of the pipe 200 to take place at a regular speed over theentire copper laminate 2 or at least over the portions of the copperlaminate 2 which are sensitive to uneven etching. For this reason, theend positions 208, 210 are located immediately outside the cathode 12 sothat the pipe 200 will have time to reach full and regular speed afterturning before the pipe 200 passes over the sensitive portions of thecopper laminate 2. The electrolyte flow supplied by the pump 212 to thepipe 200 is passed through a nozzle 214 towards the second flat side 204of the cathode 12, passed through the cathode 12 and flushed over theflat side 6 of the copper laminate 2, as shown by arrows in FIG. 4 b,whereby even and efficient etching is achieved. The nozzle 214 ends at adistance H3 of about 10 mm from the second flat side 204 of the cathode12.

It is important for the electrolyte flow to be even along the length ofthe pipe 200. This is why a flow-restricting plate 216 is arranged inthe pipe 200, as shown in FIGS. 4 b and 4 c. The flow-restricting plate216 decreases the cross-section of the pipe 200 along its length, seenfrom an inlet 218 for the electrolyte flow. As shown by arrows in FIG. 4c, the electrolyte flow is introduced through the inlet 218 and leavesthe nozzle 214 in the form of an even electrolyte flow.

The cathode 12, the copper laminate 2 and the pipe 200 are all immersedin the electrolyte. To provide an electrolyte flow, which can be passedthrough the electrolyte and the cathode 12 towards a flat side 6 of thecopper laminate 2, it is, under these circumstances, necessary for theelectrolyte flow leaving the nozzle 214 to be substantially laminar.FIG. 4 d, which shows the area IVd in FIG. 4 b on a larger scale, showsa way of providing such a laminar electrolyte flow. In one wall 220 ofthe pipe 200, a first opening is provided in the form of a first gap 222extending along the pipe. The gap 222, which has a width of 1.5 mm, endsin an antechamber 224, which is formed of a longitudinal casing 226 andhas an inner width IB of about 20 mm and an inner IL of about 30 mm. Thecasing 226 has a second opening in the form of a second gap 228, whichis located just opposite the opening 222. The second gap 228 has a firstand a second wall 230 and 232, respectively. The walls 230, 232 arecompletely parallel to each other and have a length L of 5 mm. Thedistance between the walls 230, 232, i.e. the width W of the gap 228, is1.5 mm. The extension LS of the second gap 228 along the pipe 200 issuitably greater than the width of the copper laminate 42 over which thepipe 200 is displaced. This is because the flow closest to the endportions 234 and 236 of the gap 228 is not even enough to obtain goodetching of the copper laminate 42.

FIGS. 4 e-4 f show an alternative embodiment of the pipe 200 shown inFIGS. 4 a-d in the form of a pipe 250. The pipe 250 has a device formoving the pipe 250 (not shown in FIG. 4 eof the same type as the rail206 shown in FIG. 4 a. The pipe 250 has a first row of nozzles 254 and asecond row of nozzles 256 in the wall 252 located closest to thecathode. The pressure drop in the nozzles 254, 256 makes it unnecessaryto provide a flow-restricting plate to obtain an even electrolytepressure along the pipe 250 in the embodiment shown in FIGS. 4 e-f.

The nozzles 254 in the first row and the nozzles 256 in the second roware in staggered relationship transversely to the direction of movementP of the pipe 250 such that a flat side 6 of the copper laminate 2 iswell covered during the reciprocating motion of the pipe. 250 over thesecond flat side 204 of the cathode 12. The nozzles 254 and 256 arecircular and their openings are located at a distance H4 of 10 mm fromthe second flat side 204 of the cathode 12. Each nozzle 254, 256 has afirst opening in the form of a circular inlet hole 258, which has adiameter of 12 mm, an antechamber 262 and a second opening in the formof a circular hole 264 formed in the wall of the antechamber 262 closestto the cathode 12. Each of the circular holes 264 has the form of astraight, circular cylinder with a diameter of 3 mm and a length of 5 mmand has been formed in a manner similar to that described above inconnection with the second gap 228 of the pipe 200. The circular nozzles254, 256 provide a good electrolyte flow along the flat side 6 of thecopper laminate 2, since the jet, which has a substantially laminarflow, will be spread from an individual nozzle 254, 256 in alldirections when it hits the flat side 6 of the copper laminate 2.

FIGS. 5 a-5 b show another embodiment of a flow-generating means in theform of a bar formed as a scraper 300. The scraper 300, which is a solidrectangular sectional element made of acid proof steel, is attached to asupporting arm 302 fixed on a driving motor 304. The motor 304 can bedriven along a rail 306 and can thus move the scraper 300 over thecathode 12 between the end positions 208, 210 in the manner describedabove with reference to FIG. 4 a-f. Two longitudinal wall plates 308 and310 constitute walls surrounding the space in which the scraper 300 ismoved. In its portion located closest to the cathode 12, the scraper 300has a sealing strip 312 made of an electrically nonconductive material,such as rubber. The sealing strip 312 seals the space between thescraper 300 and the second flat side 204 of the cathode 12.

When the scraper is moved in the direction of the arrow P at a suitablespeed, a pressure wave 314 forms before the scraper 300. Consequently,the liquid surface 316 immediately before the scraper 300 will besituated higher than the liquid surface 318 immediately behind thescraper 300. As a result, the pressure wave 314 will generate anelectrolyte flow, illustrated by arrows in FIG. 5 b, through the cathode12 towards a flat side 6 of the copper laminate 2, yielding even andefficient etching. Behind the scraper 300, the electrolyte flow willflow back up through the cathode 12. The height HS of the scraper 300 is100 mm. The cassette 1 is placed horizontally in an electrolyte bath.The cathode 12 is placed about 50 mm below the liquid surface formed bythe electrolyte when the scraper 300 is not being moved. Depending onthe desired speed of the motion of the scraper 300 and the desiredelectrolyte flow through the cathode 12, the height HS of the scraper300 can be selected in the range of about 20-250 mm, the cathode 12being placed at a suitable distance below said liquid surface of theelectrolyte, for example about 10-150 mm below the same.

The above-described scraper 300 can also be used when both the cassette1 and the scraper 300 are completely immersed in the electrolyte, suchas when the cassette 1 is placed vertically in an electrolyte bath.

In such a case, a covering plate (not shown) has to be used, which isparallel to and substantially covers the cathode 12 and which isarranged at the other side, relative to the cathode, of the supportingarm 302 in the vicinity thereof, in order to generate the surface effectcreated by the liquid surface 316 in the embodiment described above.

FIG. 6 shows yet another embodiment of a flow-generating means in theform of a double scraper 400. The scraper 400 has a first blade 402provided with a first rubber moulding 404 and a second blade 406provided with a second rubber moulding 408. The blades 402, 406 arefixed on two opposite sides of a pipe 410. A supporting arm 412 isattached to the pipe 410. The supporting arm 412 is in turn attached toa combined pumping and driving unit 414 arranged on a rail 416. The unit414 is arranged to move the scraper 400 reciprocatingly over the secondflat side 204 of the cathode 12 and to pump an electrolyte flow into thepipe 410. The pipe 410 is equipped with a flow-restricting plate 418 toprovide an even electrolyte pressure along the pipe 410. The pipe 410has a longitudinal gap 420, out of which the electrolyte flows downthrough the cathode 12. When moving in the direction of the arrow P, thedouble scraper 400 generates a pressure wave 422 before itself in amanner similar to that described above with reference to FIGS. 5 a-b.The pressure wave 422 generates a flow (in FIG. 6 illustrated by arrowsdirected obliquely downward) through the cathode 12 and towards a flatside 6 of the copper laminate 2. The scraper 400 also generates asubstantially laminar electrolyte flow, which leaves the pipe 410through the gap 420 and is passed through the cathode 12 towards theflat side 6 of the copper laminate 2. The combination of the effectsdescribed with reference to FIGS. 5 a-b and FIGS. 4 a-f results in thedouble scraper 400 generating excellent stirring of the electrolyte atthe flat side 6 of the copper laminate 2.

FIG. 7 shows a further embodiment of a flow-generating means in the formof a roller 500. The roller 500 is arranged to be rolled over the secondflat side 204 of a cathode 12. A rail 502 is arranged to move the roller500 over the second flat side 204 of the cathode 12 under rotation, asshown by an arrow R. When quickly moving the roller 500 over the cathode12 in the direction of the arrow P, a pressure wave 504 is generatedbefore the roller 500. The pressure wave 504 generates an electrolyteflow (in FIG. 7 represented by arrows directed obliquely downward)through the cathode 12 and towards a flat side 6 of the copper laminate2. The roller 500 is suitably made of an electrically insulatingmaterial, such as PEEK or polypropylene.

It will be understood that the flow-generating means shown in FIGS. 4-7can also be used when a copper laminate 42 is to be etchedsimultaneously on both flat sides 46, 52, such as is the case, forexample, with the cassettes 40 and 100, respectively. In this case, afirst flow-generating means 200, 250, 300, 400, 500 is arranged at asecond flat side of the first cathode 58, 102 facing away from thecopper laminate 42, The first and second flow-generating means 200, 250,300, 400, 500 is arranged at a second flat side of a second cathode 66,104 facing away from the copper laminate 42. The first and the secondflow-generating means can then be moved synchronously or independentlyof each other.

The flow-generating means 200, 250, 300, 400, 500 is suitablyelectrically insulated from the cathode 12, 58, 102. This insulationcan, as already described, either consist in the flow-generating means200, 250 being located at a distance from the cathode or in a part ofthe flow-generating means 300, 400, 500 that is in contact with thecathode 12, 58, 102 being made of an electrically insulating material.

In case of very large copper laminates, several separate flow-generatingmeans can be moved over the same cathode 12, 58, 102 to provide goodcoverage of the copper laminate without necessitating too high a speedof the motion of the flow-generating means.

It has already been described how the cassette 1, 40, 100 is keptstationary while the flow-generating means 200, 250, 300, 400, 500 ismoved over the respective cathodes 12, 58, 66, 102, 104. It is alsopossible to move simultaneously both the cassette 1, 40, 100 and theflow-generating means 200, 250, 300, 400, 500, or to move the cassette1, 40, 100 and keep the flow-generating means 200, 250, 300, 400, 500stationary. The latter alternatives can be used in a continuous process,in which the cassette 1, 40, 100 is moved through a number of processingsteps by a conveyor. The cassette 1, 40, 100 can be moved eitherhorizontally, i.e. in a lying position, or vertically, i.e. in anupright position.

The flow-generating means 200, 250, 300, 400, 500 is suitably movedrelative to the cathode 12, 58, 66, 102, 104 in the direction of thearrow P at a speed of 0.01-1 m/s, preferably about 0.02-0.5 m/s.

To ensure a substantially laminar flow out of the nozzle 214, 254, 256,the length L of the second opening 228, 264 should be greater than thewidth W, D. The width W and the diameter D of the second opening 228,264 should be 0.4-10 mm, preferably 0.8-5 mm. The flow velocity in thesecond opening 228, 264 should then be about 2-25 m/s, preferably 7-14m/s.

The electrolyte flow supplied to the pipe 200, 250 should correspond toa total electrolyte flow of 5-500 l/(s*m²), preferably 15-200 l/(s*m²).When, for example, the pump 212 is pumping an electrolyte flow of 8 l/sto the pipe 200 and a first flat side 6 of the copper laminate 2 has anarea of 0.45 m×0.30 m=0.135 m² , a total electrolyte flow of 8/0.135=59l/(s*m²) is obtained. Locally under the nozzle 214, the electrolyte flowwill, of course, be considerably higher.

The distance H between the copper layer 8 of the copper laminate 2 and afirst flat side 202 of the cathode 12 is suitably 100 mm at most. Whenetching copper laminates 42 with thin electric conductors, i.e. when theelectric conductors that remain under the resist layer 10 after thesurrounding portions of the copper layer 8 have been etched away have awidth of about 200 μm or less, the distance H should be about 20 mm atmost, preferably about 10 mm at most (the same applies to the distancesH1 and H2 in double-side etching). When etching copper laminates 42 withelectric conductors having a width of about 50 μm and less, even smallerdistances H, H1, H2, for example, of about 5 mm and less, may beapplicable. The distance H3, H4 from the opening 228, 264 of the nozzle214, 254, 256 to the second flat side 204 of the cathode 12 is suitablyabout 50 mm at most, preferably about 25 mm at most, and morepreferably. about 5-15 mm.

FIG. 8 schematically shows a device for etching of a copper laminate 42.The copper laminate 42 is fixed in a cassette 100 of the above-describedkind. A first pipe 200 of the above-described kind is reciprocatinglymoved by means of a rail 206 over the first cathode 102 and generates anelectrolyte flow through the cathode 102. A second pipe (not shown inFIG. 8) generates in a corresponding manner an electrolyte flow throughthe cathode 104. The cassette 100 and the pipe 200 are placed in anetching tank 600 and located below the liquid surface 602 of theelectrolyte. A voltage generator 604 supplies negative voltage throughcathode lines 148 to the cathode frames 106 of the cassette 100 andpositive voltage through anode lines 152 to the anode frames of thecassette 100. A pump 606 pumps electrolyte through liquid conduits 608,610 to a regeneration tank 612. The regeneration tank 612, which isequipped with an agitator 614, is used to regenerate the electrolyte soas to maintain the etching capacity of the device. An overflow pipe 616conducts regenerated electrolyte back to the etching tank 600.

As seen in FIG. 8 a measuring device 618 is arranged to measure theetching capacity of the electrolyte. The measuring device 618 can beintended for either manual or automatic measuring. The measuring device618 can, for example, analyse the content of an etching compound in theelectrolyte. The measuring device 618 transmits a signal to a dosingdevice 620. The dosing device 620 controls the amount of regenerationchemicals which is to be pumped by a pump 622 to the regeneration tank612 from a dosing tank 624 through conduits 626, 628. The regenerationtank 612 also has a conduit 630 for feeding water and additives, if any,to the electrolyte and a conduit 632 for evacuating electrolyte to avoidaccumulation of undesirable substances in the electrolyte. Both theetching tank 600 and the regeneration tank 612 are equipped with anelectric heater 634 and 636, respectively, to provide the desiredetching temperature.

It will be understood that a number of modifications of the embodimentsdescribed above are conceivable within the scope of the invention.

Thus, the cathode 12, 58, 66, 102, 104 can be designed in many differentways. Besides various types of netting and metal fabric, also perforatedplates and bars, which are placed at a distance from each other, can beused as a cathode. The important thing is that the cathode has goodelectric conductivity and good permeability to the electrolyte flowwhile allowing etching without any undesirable patterns being generatedon the board for electric reasons.

To obtain increased durability against the electrolyte, the cathode cansuitably be plated with a precious metal, for example gold.Correspondingly, the parts of the cassette which do not need to havegood conductivity can be protected against the electrolyte, for exampleby a chromium oxide coating or an epoxy coating.

The flow-generating means 200, 250, 300, 400, 500 can be moved relativeto the cathode 12, 58, 102 in various motions, such as rotating,oscillating and sweeping in a curved motion, such as a circular motion.However, the reciprocating motion described above and the sweepingmotion, which is to be considered as a kind of reciprocating motion,yield an optimal etching result. The displacing means can, for example,comprise a splined shaft, a stepping motor or a conveyor belt or chainwith a variable direction of movement. The distance H3, H4 can suitablybe 0.5-10 cm.

The insulators 16, 62, 70, 122, 124 can be made of a number of differentelectrically insulating materials. Use can, for example, be made ofepoxy adhesive, teflon and PEEK. The important thing is that theinsulator should have a precise and well-defined thickness to make thedistance H, H1 and H2 precise over the entire flat sides 6, 46, 52 ofthe board 2, 42. The anode frames 18, 64, 72, 114, 116 also have to havea precise and well-defined thickness. Furthermore, distances H, H1, H2of 10 mm and less require the anode frames to be made very thin. Theelectric insulator should be arranged to keep the anode frame at a veryshort distance from the cathode and the cathode frame, but not at such ashort distance as to provoke a short circuit. With the above-mentionedshort distances H, H1, H2, high demands have to be placed on theflatness of the anode frames, cathode frames, the cathodes and theinsulators. However, the divisible cassette 1, 40, 100 allows efficientetching of boards 2, 42 also when applying these short distances H, H1,H2. In particular the cassette 100, which has two parts 130, 132, eachof said parts comprising a cathode 102, 104, a cathode frame 106, 108,an insulator 110, 112, and an anode frame 114, 116, which are attachedto each other, and said parts being interconnected by means of hinges134, 136, is very efficient in these cases.

The flow-generating means 200, 250, 300, 400, 500 can also be used ifthe cathode and the board are not attached to each other, such as incontinuous processes, in which boards are conveyed under cathodes forexample on rollers, which contact the board which then becomes an anode.The flow-generating means can be fixed or movable and generate anelectrolyte flow through the cathode towards the board. However, theabove-described cassettes 1, 40, 100 are those that are mostadvantageous.

The cassette 1, 40, 100 can also comprise a solid cathode, which forexample is made of a non-perforated metal sheet. In such a cassette,there is either no exchange of electrolyte whatsoever during etching, oran electrolyte flow is provided through holes in the sides of thecathode frame/frames, the electrolyte flow being passed along thesurface of the board. It is, however, most advantageous for the cathodesto be perforated by openings, as described above.

Also layers of metal other than copper can be etched using theabove-described methods and devices. Examples of such metal layers arechromium and nickel layers. The invention is, however, particularlyconvenient in the etching of thin copper layers. The thickness of theelectrically conductive layer can be about 0.1 μm to about 1000 μm. Whenetching copper layers, the thickness of the layer is usually about 5-35μm.

The substantially laminar electrolyte flow through the cathode issuitably directed perpendicularly to the flat-side of the board. It is,however, also conceivable in some applications, where appropriate, touse a pipe with nozzles directing the electrolyte flow at a differentangle to the flat side of the board. When using at least two rows ofcircular nozzles 254, 256, the nozzles in the two rows can, for example,be directed in different directions, which provides a conical spraypattern under the pipe.

The cassette 1, 40, 100 can be kept together with the aid of variousmeans. Besides the above-mentioned magnets and clamps, use can be madeof insulating bolts, insulating fasteners, etc. If, for example, anelectrically insulating supporting frame 20 is used, or if two cathodes58, 66 are to be connected to the same voltage source, also electricallyconductive, clamping means, such as steel clamps, can be used to keepthe cassette pressed together, provided that these means do not get intocontact with the anode frames or the board.

As mentioned above, the cassette 1, 40, 100 can be used in both avertical and a horizontal position and, naturally, at any angletherebetween. The cassette 1, 40, 100 can also be used in batchwisemethods, as described above reference to FIG. 8, and in continuousmethods. In a continuous method, the cassette is conveyed betweendifferent processing stations by a conveyor, which may for examplecomprise rollers, endless belts or guides.

1. A method of etching copper on boards, comprising: applying anelectric voltage between a cathode and a board, which constitutes ananode, the cathode and the board being immersed in an electrolyte,wherein: the electrolyte comprises at least a first component, which canbe reduced from a first state in the form of an ion comprising a metalatom with a first positive oxidation number, to a second state in theform of an ion comprising the metal atom with a second positiveoxidation number, the second positive oxidation number being smallerthan the first positive oxidation number; the electrolyte includes afirst redox potential for reducing the first component from the firststate to the second state and a second redox potential for reduction ofdivalent copper ions to metallic copper, the first redox potential beinggreater than the second redox potential; and metallic copper on theboard is oxidized and converted into positively charged copper ionswhile the first component is reduced from its first state to its secondstate, wherein precipitation of the copper ions on the cathode does notoccur.
 2. The method as claimed in claim 1, wherein the first componentis selected from the group consisting of Fe, Mn, Ce and inorganic andorganic complexes thereof.
 3. The method as claimed in claim 2, whereinthe first component is Fe, which is added to the electrolyte in the formof Fe³⁺ or a precursor thereof.
 4. The method as claimed in claim 1,wherein the capacity of the electrolyte to etch the copper on the board,in the absence of an applied voltage, corresponds to an etching rate of6-100 nm/s.
 5. The method as claimed in claim 1, wherein the electrolytefurther comprises a second component that can be reduced from a firststate in the form of an ion having an atom with a first positiveoxidation number, to a second state in the form of an ion comprisingsaid atom with a second positive oxidation number, which is smaller thansaid first positive oxidation number of the second component, and athird redox potential in the electrolyte for reduction of the secondcomponent from its first state to its second state being lower than thesecond redox potential in the electrolyte for reduction of divalentcopper ions to metallic copper.
 6. The method as claimed in claim 5,wherein the second component is Sn.
 7. The method as claimed in claim 1or claim 5, wherein the electrolyte further comprises fluoride ions, F⁻in a concentration of 0.01-0.5 mol/I.
 8. The method as claimed in claim1 or claim 5, wherein the electrolyte further comprises chloride ions,Cl⁻, in a concentration of 0.03-1.5 mol/I.
 9. The method as claimed inclaim 1, wherein the voltage applied is 0.5-6 V.
 10. The method asclaimed in claim 1, wherein the concentration of the first component is0.02-0.7 mol/I.
 11. The method as claimed in claim 1, wherein at leasttwo different first components are added to the electrolyte.
 12. Themethod as claimed in claim 1, wherein the pH of the electrolyte is −0.5to 4.0.
 13. The method as claimed in claim 1 or claim 5, wherein theelectrolyte is treated with a regenerant chosen from strong oxidizingagents for converting the first component from its second state to itsfirst state.
 14. The method as claimed in claim 1 or claim 5, wherein afourth redox potential in the electrolyte for reduction of said secondstate of the first component to a third state is lower than the redoxpotential of hydrogen gas.
 15. The method as claimed in claim 6, whereinthe Sn is present in the electrolyte in a concentration of 0.005-0.4mol/I.
 16. The method as claimed in claim 15, wherein the electrolytefurther comprises fluoride ions, F⁻, in a concentration of 0.01-0.5mol/I.
 17. The method as claimed in claim 13, wherein the regenerantcomprises Na₂S₂O₈.